If $5a + 8b + 5c = -4$ and $x + 2y = 5$, what is $24b + 15a - 14y - 7x + 15c$ ?
Solution: $= 15a + 24b + 15c - 7x - 14y$ $= (3) \cdot (5a + 8b + 5c) + (-7) \cdot (x + 2y)$ $= (3) \cdot (-4) + (-7) \cdot (5)$ $= -12 - 35$ $= -47$